Derivation of bonding distance and vibration frequency from field evaporation experiments

This paper extends the analysis of previously reported data, concerning the field evaporation of rhodium atoms that escape as Rh⁺ and are then post-ionised to Rh²⁺. A more general formula is derived for obtaining values of critical distance x^cr from correlated measurements of temperature, field (F), activation energy (Q), and onset appearance energy. A zkT-type correction is included. If prior to evaporation the surface atom is vibrating in an effectively parabolic potential well, and a Gomer-type escape mechanism operates, then experimental plots of $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ versus $\text{1}\text{F}$ and of x^cr versus $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ should be linear. Such plots may be used to obtain estimates of zero-Q evaporation field F^e, electrical bonding distance a, and vibrational force-constant k and frequency v. These formulae and methods are applicable to a range of materials and evaporation situations. For rhodium this paper establishes that escape takes place via a Gomer-type escape mechanism, and deduces the following values: F^e = 61 ± 20 V/nm; a = 0.13 ± 0.035 nm; v = (1.4_+0.5^?0.3) × 10¹²Hz.     

Type I FA (Rb, Cs) and II FA (Li, Na) tunable laser activities and donor-acceptor properties of O and O at KCl (001) surface: first principle calculations

Type I F_A (Rb⁺, Cs⁺) and II F_A (Li⁺, Na⁺) tunable laser activities, adsorptivity and donor-acceptor properties of O and O⁻ adsorbates at the flat surface of KCl crystal were investigated using an embedded cluster model and ab initio methods of molecular electronic structure calculations. Ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field of the host surface, and the nearest neighbor ions to the defect site were allowed to relax to equilibrium. Based on the calculated Stokes shifted optical transition bands, F_A tunable laser activities were found to be inversely proportional to the size of the dopant cation (Li⁺, Na⁺, Rb⁺, Cs⁺) relative to the host cation (K⁺). This relation was explained in terms of the axial perturbation of the impurity cation. The probability of orientational bleaching attributed to the RES saddle point ion configuration along the 〈110〉 axis was found to be inversely proportional to the size of the dopant cation, with activation energy barriers of ca. 0.44-3.34 eV. Surface relaxation energies of type II F_A centers were more important than those of type I F_A centers. In terms of defect formation energies, the products of type II F_A center imperfection were more stable than those of type I F_A. The difference between F or F_A band energies and exciton bands depended almost exclusively on the size of the positive ion species. As far as the adsorptivity of O and O⁻ is concerned, the results confirm that surface imperfection enhances the adsorption energies by ca. 4.38-16.37 eV. O and O⁻ penetrate through the defect-containing surface. The energy gap between the adsorbate and the defect containing surface and the donor-acceptor property of adsorbate play the dominant role in the course of adsorbate substrate interactions and the results were explained in terms of electrostatic potential curves and Mulliken population analysis.     

Discriminative deep inelastic tests of strong interaction field theories

It is demonstrated that recent measurements of ∫¹₀F₂(x, Q²)dx eliminate already all strong interaction field theories except QCD. A detailed study of scaling violations of F₂(x, Q²) in QCD shows their insensitivity to the gluon content of the hadron at presently measured values of Q².     

Anisotropic photoemission and chemisorption-bond geometry on Ge(111) and Si(111) surfaces

In order to study the effects of photon-polarization selection rules on chemisorptionbond geometry, we have measured photoelectron spectra as a function of angle of incidence, θ_i, in the range 28° ? θ ? 80°. A noble-gas UV resonance lamp and cylindrical mirror analyzer were used to measure both bonding and non-bonding surface Orbitals. A large enhancement (200–400%) of the photocmission is found when photon electric field intensity is near the maximum normal to the surface. This indicates a spatial variation of microscopic fields which is approximately independent of adsorbate bonding since it is determined by the optical properties of the substrate. In addition, we observe some effects on adsorbate photoelectron peaks due to different orbital symmetry. The case of atomic hydrogen chemisorption is discussed as an example of this latter effect.     

Interplay between a homogeneous magnetic field and an antiferromagnetic field in one-dimensional superconductors

Mean-field theory applied to superconductors with one-dimensional band in the presence of both the homogeneous magnetic field H₀ and the antiferromagnetic field H_Q, the second-order phase transition temperature is investigated for the arbitrary angle θ between H₀ and H_Q. It is found that the remarkable superconducting region in the case of θ = 0 is retained only for small θ and that the spatially dependent order parameter coexists with the spatially uniform order parameter except for θ = τ/2.     

Adsorption sites of111In and111mCd on Pd(111) surfaces characterized by the electric field gradient

PAC measurements of the electric field gradient at the nucleus of isolated probe atoms on Pd(111) surfaces lead to the identification of five adsorption sites successively occupied by the parents¹¹¹In and^111mCd during annealing between 80K and 600K. The data are consistent with an estimated activation energy for Pd surface self diffusion of E_a=0.76(8)eV.     

Crystal face specificity of xenon adsorption on iridium field emitters

Fundamental problems of the adsorption of noble gas atoms on metal surfaces are discussed on the basis of new data of xenon adsorption on well-defined crystal faces of iridium. These data include surface potentials ( = changes in work function), heats of adsorption and their decrease with increasing coverage; they have been obtained by using a field emitter probe-hole assembly. It is found that the heat of adsorption Q_hkl is not simply additive in the number of Ir atoms contacting a Xe atom on a given site; in particular for the close-packed faces, Q₁₁₁ and Q₁₀₀ are relatively too high. Apparently, strong bonding is favoured by high work function of the adsorbing crystal face. This proves a significant contribution of a charge-transfer no-bond interaction to the adsorption bond. A model of Xe polarization by an electric surface field is rejected, as it predicts the wrong sign for the adsorption dipole. While at low coverage adsorption is confined to sites determined by the atomic topography of the adsorbing surface, several possibilities exist for high coverages. Either a two-dimensional close-packed layer is formed with little or no epitaxial relation to surface topography, or adsorption remains confined to certain sites. The present data favour the former possibility for atomically smooth faces in agreement with recent LEED results. For atomically rough faces however, the smallness of the decrease of Q_hkl with coverage seems to favour site adsorption even at high coverage. The latter result is of relevance for surface area determinations by means of “physical” adsorption.     

Determination of the242m Am nuclear moments

The hyperfine structure of Am atoms was investigated in an atomic beam by laser spectroscopy. The observed splittings were evaluated with respect to the magnetic dipole and electric quadrupole moments of^242m Am. The results are: μ _I (^242m Am)=+0.97(5)nm,Q(^242m Am)=+6.5(2.0)b.     

Measurement of the surface density of states by field ionization

It is shown that recent measurements of field ion energy distributions from clean tungsten surfaces probe the density of metal states in the vicinity of the surface. We find j(ω) = (2π/kh)Σ_m| ∫ d³rψ_m(r)γ_z|²δ(ω??_m), where j(ω) is the ion current a ω, ψ_mand ?_m are electronic metal eigenfunctions and eigenvalues in the presence of the external electric field used in field ionization and γ(z) is a function which is large near the noble gas atom. An explicit expression for γ(z) is given in the text. It is estimated that tungsten metal states with values of k₆ at least as large as 0.5 Å^?1 make an appreciable contribution to j(ω) where k₆ is the electron momentum parallel to the surface.     

The effect of high pressure on the electric field gradients at two inequivalent lattice sites of tantalum in ω-zirconium

The pressure variation of the electric field gradients at substitutional tantalum atoms on the two lattice sites (A and B) of ω-zirconium have been determined to be (? lnq ^A/?P)_T = ?1.05(26)10^?3 kbar^?1 and (? lnq ^B/?P)_T = +6.03(40)10^?3 kbar^?1. In addition the thermal expansion coefficients of the unit cell of ω-Zr were found to be α_‖ = +3.5(3)10^?6 K^?6 and α_⊥ = +7.9(4)10^?6 K^?1. A combined analysis of the pressure dependence and the previously determined temperature dependence of the electric field gradients supports the assumption of two different bonding types at the A and B sites.     

F2+ defect based model for tunable laser activity and interaction of atomic halogens (F,Cl and Br) at the low coordinated surfaces of LiH: ab initio calculations

The twofold potential of F₂⁺ color center at the low coordinated surfaces of LiH in providing tunable laser activity and adsorption properties for atomic halogens is examined using ab initio calculations. Based on Stokes shifts of the examined clusters, the F₂⁺ laser activity was very weak, but increases significantly as the coordination number decreases from 5 (flat) to 4 (edge) to 3 (H⁻ corner). The corner site is the least probable for relaxed excited state orientational destruction of F₂⁺. The exciton (energy) transfer takes place from the corner to the flat or edge sites. The Glasner–Tompkins relation is generalized to include the low coordinated surfaces of LiH. The F⁺₂ color center change the nature of halogen–surface interaction from physical adsorption to chemical adsorption. The halogen–surface interactions were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred between the halogen and the surface. The “covalent spin pairing” mechanism play the dominant role in the course of adsorbate–substrate interactions.     

The electric field gradient at188Os nuclei in Re metal

The electric quadrupole interaction of the first excited 2⁺ state of¹⁸⁸Os in hexagonal rhenium metal was investigated by means of the time-differential perturbed angular correlation technique. From the observed quadrupole frequencyV _Q=170(7) MHz, we deduce an electric field gradient value of |V_zz|=4.77(23)·10²¹V/m² for the system OsRe. The half-life of the 2⁺ state was measured to be 641(4)ps.     

锐钛矿型TiO2(101)面原子几何及弛豫结构的第一性原理计算

采用平面波超软赝势方法计算了锐钛矿型TiO₂(101)面的表面能和表面原子弛豫结构.首先对TiO₂(101)面的6种不同的表面原子终止结构的体系总能量进行了计算,结果表明终止原子为两配位的O原子、次层为五配位的Ti原子的表面结构最为稳定.针对该表面研究了表面能和原子弛豫与模型中原子层数和真空厚度的关系,当原子层数为12层,真空厚度为0.4nm时,表面能收敛度小于0.01J/m².研究发现:表面上两配位的O原子向里移动约0.0012nm,五配 关键词： 第一性原理 2')" href="#">TiO₂ 表面结构 弛豫     

Dilation analyticity in constant electric field

The resolvent of the operatorH ₀(ε, θ)=?Δe ^-20+εx ₁ e ^θ is not analytic in θ for θ in a neighborhood of a real point, if the electric field ε is non-zero. (One manifestation of this singular behavior is that for 0<|Im θ|<π/3,H ₀(ε, θ) has no spectrum in the finite plane.) Nevertheless it is shown that the techniques of dilation analyticity still can be used to discuss the long-lived states (resonances) of a system described by a Hamiltonian of the formH=?Δ+εx ₁+V(x).     

High field microscopy of nickel on tungsten

Field emission and field ion microscopy have been used to study the properties of nickel layers adsorbed on tungsten, and the growth of nickel crystallites. The first monolayer of nickel has a maximum density of 0.97 ± 0.05 × 10¹⁹ atoms m^?2 and results in an increase in the work function which can be attributed to the formation of dipoles of moment μ₀ = 1.70 ± 0.08 × 10^?30Cm at zero coverage and polarizability $\text{α = 7.3 ± 0.05}\text{A}\text{?}{}^3$. Nickel desorbs from the tungsten surface with activation energy 4.22 ± 0.01 eV and second layer atoms desorb with activation energy 3.2 ± 0.02 eV. Surface diffusion of second and higher layers over clean tungsten layer is believed to proceed by the “unrolling carpet” mechanism, with activation energy 0.93 ± 0.03 eV in close agreement with measurements of surface self-diffusion of nickel. Nickel does not dissolve appreciably in single-crystal tungsten and we confirm that atomic disordering at the nickel-tungsten interface is confined within a few angstroms of the interface. Well-ordered crystallites can be grown from a central nuclear structure which develops on (110)W. Combination of field ion and field emission techniques indicate that the crystallites adopt the expected growth form, having surfaces comprising large low-index faces, and also serve to confirm that field emission images alone cannot be relied upon to give an indication of crystallite shape. Crystallites invariably form upon an adsorbed layer which is at least one atom thick but may be thicker depending upon conditions of growth. The growth of crystals in situ offers the possibility of generating well-ordered low-index planes of large area which are suitable for further study, but it has yet to be confirmed that they behave as surface planes of bulk nickel.     

Measurement of the magnetic formfactor of the deuteron forQ 2=0.5 to 1.3 (GeV/c)2 by a coincidence experiment

Elastic electron deuteron scattering was measured at the Bonn 2.5 GeV Electron-Synchrotron using a two spectrometer coincidence technique. Data were taken at values of the four momentum transfer squaredQ ²=0.50, 0.60, 0.78, 1.00, and 1.3 (GeV/c)² at electron scattering angles θ _e =77.5° to 139.0° in order to separate the magnetic dipole formfactorF _M (Q ²) from the electric formfactorF _E (Q ²). Experimental cross sections are compared to theoretical predictions and to measurements of other laboratories. The results indicate that contributions from meson exchange currents are not negligible in thisQ ² region.     

The electric field gradient at the nuclear site of177Lu in Zn and In

The electric quadrupole interaction frequencyν _Q =eQV _zz /h of¹⁷⁷Lu in single crystals of Zn and In has been measured by the method of low temperature nuclear orientation. The results are $$\begin{gathered} v_Q ({}^{177}Lu\underline {Zn} ) = - 180(5)MHz \hfill \\ v_Q ({}^{177}Lu\underline {In} ) = - 19(5)MHz. \hfill \\ \end{gathered} $$ With the known quadrupole moment of¹⁷⁷LuQ=3.39 (2) b we derive for the electric field gradientV _zz (Lu Zn)=?2.20 (5)×10¹⁷ V/cm² andV _zz (Lu In)=?0.23 (6)×10¹⁷ V/cm². The results are compared with magnetostriction measurements of silver single crystals doped with rare earth atoms.     

Carbon monoxide and carbon dioxide interaction with tantalum

The adsorption of carbon monoxide and carbon dioxide on tantalum and the dissolution of these gases in the adsorbent at T ? 300 K have been studied. The flash-filament method (FFM) in a monopole mass-spectrometer and a field emission microscopy was used in the same apparatus. Carbon monoxide and carbon dioxide dissociate on the tantalum surface, carbon monoxide being desorbed in both cases during the flash. The desorption curves of CO reveal three different binding states: two of them (α and $\text{}\text{?}$gb₁) for the adsorbed particles whereas the high temperature desorption state relates to the adsorbate dissolved in the metal, For the $\text{}\text{?}$gb₁ state of CO the activation energy, the pre-exponential factor and the kinetic order in the kinetic equation of desorption have been estimated. They turned out to be E = 110 kcal/mol, C = 3 × 10¹²sec^?1, and ν = 1. The activation energy of diffusion for CO in tantalum and the energy of outgassing for the metal were found to be 9.4 and 49 kcal/mole, respectively.     

Une détermination du coefficient d'auto-diffusion de surface du molybdène

A recent study on the morphological evolution of a metal tip has shown that free evaporation must have a vital influence on the determination of the surface self-diffusion coefficient D_s in the high temperature region. We now present a method which enables the experimental determination of D_s: (1) in the presence of an evaporation, and (2) in a relatively great region of temperature. As an example, D_s of molybdenum has been measured. We obtain: (1) activation energy Q_s = 48.3 kcal/mole and diffusivity D_o = 0.3 cm²/sec for T < 0.6T_F (T_F melting temperature), and (2) Q_s = 76.7 kcal/mole and D_o = 690 cm²/sec for 0.65 T_F < T < 0.85 T_F; which indicate a change of the diffusion mechanism at about 0.65 T_F.